add a guide to swap

[coreos]

add a guide to swap

[coderfi]

FYI, to disable COW, I followed the instructions from: https://wiki.archlinux.org/index.php/Btrfs

So this is what I did:

sudo fallocate -l 2048m /2GiB.swap
sudo chmod 600 /2GiB.swap
sudo chattr +C /2GiB.swap
sudo mkswap /2GiB.swap
# ... added the swap.service unit to /var/lib/coreos-install/user_data ...
sudo reboot

After the reboot, I took a look at top and saw the swap active and used

    Swap:  2097148k total,    10144k used,  2087004k free,   245512k cached

Not sure if COW was actually disabled, nor do I know the actual performance gain.

Here is the units snippet in my user_data file (per the above comments):

units:
    - name: swap.service
      command: start
      content: |
        [Unit]
        Description=Turn on swap

        [Service]
        Type=oneshot
        Environment="SWAPFILE=/2GiB.swap"
        RemainAfterExit=true
        ExecStartPre=/usr/sbin/losetup -f ${SWAPFILE}
        ExecStart=/usr/bin/sh -c "/sbin/swapon $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
        ExecStop=/usr/bin/sh -c "/sbin/swapoff $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
        ExecStopPost=/usr/bin/sh -c "/usr/sbin/losetup -d $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"

        [Install]
        WantedBy=local.target

Hope this helps.

• Fi

[ClashTheBunny]

This no longer works due to btrfs not being compatible with swap files. To make it work, I had to adjust the directions so that I created the file somewhere on btrfs (/4GiB.swap) and then used this unit file:

[Unit]
Description=Turn on swap

[Service]
Type=oneshot
Environment="SWAPFILE=/4GiB.swap"
RemainAfterExit=true
ExecStartPre=/usr/sbin/losetup -f ${SWAPFILE}
ExecStart=/usr/bin/sh -c "/sbin/swapon $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
ExecStop=/usr/bin/sh -c "/sbin/swapoff $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
ExecStopPost=/usr/bin/sh -c "/usr/sbin/losetup -d $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"

[Install]
WantedBy=local.target

It's important to have the "RemainAfterExit" or else the Stop stanzas will be executed immediately.

They say that btrfs performs poorly in this situation, but if it's all you got...

[marineam]

Should work a little better if you disable COW for the swap file when you create it.

[marineam]

You may need to disable cow before using fallocate, you can only disable
cow on zero length files and off the top of my head I don't know if
fallocate counts towards that restriction or not.
On Aug 5, 2014 3:22 AM, "coderfi" [email protected] wrote:

FYI, to disable COW, I followed the instructions from:
https://wiki.archlinux.org/index.php/Btrfs

So this is what I did:

• sudo fallocate -l 2048m /2GiB.swap
• sudo chmod 600 /2GiB.swap
• sudo chattr +C /2GiB.swap
• sudo mkswap /2GiB.swap
• added the swap.service unit to /var/lib/coreos-install/user_data
• sudo reboot
• took a look at top and saw the swap active and used Swap: 2097148k
  total, 10144k used, 2087004k free, 245512k cached

Not sure if COW was actually disabled, nor do I know the actual
performance gain.

Here is the units snippet in my user_data file (per the above comments):

units:

• name: swap.service
  command: start
  content: |
  [Unit]
  Description=Turn on swap

  [Service]
  Type=oneshot
  Environment="SWAPFILE=/2GiB.swap"
  RemainAfterExit=true
  ExecStartPre=/usr/sbin/losetup -f ${SWAPFILE}
  ExecStart=/usr/bin/sh -c "/sbin/swapon $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
  ExecStop=/usr/bin/sh -c "/sbin/swapoff $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
  ExecStopPost=/usr/bin/sh -c "/usr/sbin/losetup -d $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"

  [Install]
  WantedBy=local.target

—
Reply to this email directly or view it on GitHub
#52 (comment).

[cboettig]

Thanks for this, worked like a charm for me. Is it possible to have the commands sudo fallocate... also specified in the cloud-config file instead of having to issue these manually? I see there is no runcmd in a coreos cloud-config file...

[romaninsh]

@cboettig Here, I used the following service, which also executes fallocate: https://gist.github.com/romaninsh/118952ce61643914fb00

[joshperry]

Why not support the canonical swap configuration? http://cloudinit.readthedocs.org/en/latest/topics/examples.html#adjust-mount-points-mounted

Using this: https://gist.github.com/tsertkov/573d077012ae76443cbf but 50 lines of config just for a swapfile seems excessive.

[crawford]

Related coreos/bugs#429 (comment).

[joelchen]

Are minimum of round(sqrt(RAM)) and maximum of twice the amount of RAM from https://help.ubuntu.com/community/SwapFaq recommendations we should follow on CoreOS?

[crawford]

@joelchen most CoreOS instances don't use swap at all, so the minimum is zero. Canonical's recommended upper bound seems reasonable.

[joelchen]

@crawford What are the recommended use cases for no swap, as well as for using swap in CoreOS?

[crawford]

@joelchen it depends heavily on how you plan on using the machine and the physical characteristics of the machine. I would suggest taking a look at what Linux recommends and then going with that.

[HaleTom]

Do not try what @coderfi suggested, namely, disabling CoW to try to use a swapfile with btrfs, until you read that it is supported in the official documentation.

swap file support relies on one function that btrfs intentionally does not implement due to potential corruptions.

However:

A workaround, albeit with poor performance, is to mount a swap file via a loop device.

[philips]

Someone just mentioned losteup leads to lockups https://groups.google.com/d/msg/coreos-user/xw4aeC68k6Q/vy8Iv0jWCAAJ. Can someone please write this guide? It is clearly tripping up a lot of users on something very simple. cc @ElijahCaine

[pop]

@philips I'm writing and testing a doc now. PR should be up by the end of the day.